Thermal spraying method and apparatus

ABSTRACT

A thermal spraying method involves the creation of a coating comprising titanium wire in the presence of nitrogen. The apparatus of the invention comprises a nozzle which has a cylindrical throat, with feedstock guides which guide the feedstock wires to a point of intersection in the throat. A current is passed through the wires to cause an arc in the throat, and a nitrogen rich gas under pressure is forced through the throat, generating a spray of molten particles which is used to coat a substrate. In a variation of the method, one of the feedstock wires comprises a binder metal, which produces a coating having enhanced toughness.

[0001] THIS invention relates to a thermal spraying method for producinga hard coating on a substrate, and to thermal spraying apparatus whichcan be used for producing metallic or cermet coatings on a substrate.

[0002] Arc metal spraying is used in industry to produce coatings onsubstrates by generating an arc between feedstock electrodes. The moltenfeedstock is divided into small particles of molten material by anatomising gas jet. These molten particles are propelled by the gas jetonto the substrate to be coated. The fineness of the particles isdetermined, inter alia, by the velocity of the atomising gas jet.

[0003] It is an object of the invention to provide a thermal sprayingmethod which can be used to produce hard coatings with desirableproperties, and an alternative thermal spraying apparatus.

[0004] According to a first aspect of the invention a method of forminga coating on a substrate comprises the steps of:

[0005] providing a feedstock material containing titanium;

[0006] atomising the feedstock material in the presence of nitrogen; and

[0007] spraying the atomised material onto a substrate to form a coatingcomprising titanium nitride on the substrate.

[0008] The coating may additionally comprise oxides and carbides oftitanium.

[0009] The feedstock material is preferably atomised by generating anarc between at least two feedstock elements.

[0010] Preferably, at least one of the feedstock elements is a titaniumwire which is fed towards a point of intersection between the feedstockelements where the arc is generated.

[0011] The point of intersection is preferably located within a throatof a nozzle, the method including supplying a nitrogen rich gas underpressure to the throat of the nozzle to assist in expulsion of atomisedparticles therefrom.

[0012] The gas is preferably supplied to the throat of the nozzle at apressure sufficient to generate choked gas flow in the throat.

[0013] The gas will typically be air.

[0014] At least one of the feedstock elements may be a wire comprising ametal selected to have suitable properties as a binder of the titaniumnitride in the coating, such as nickel.

[0015] According to a second aspect of the invention there is providedthermal spraying apparatus comprising:

[0016] a nozzle defining a throat having an inlet and an outlet;

[0017] at least first and second guides arranged to guide respectivefeedstock wires via the inlet towards a point of intersection in thethroat, so that connection of the wires to a power supply causes an arcin the throat between the wires, creating molten particles which areexpelled from the outlet.

[0018] The throat may comprise a tubular bore which substantiallysurrounds the point of intersection of the two feedstock wires.

[0019] The diameter of the throat is preferably substantially constantalong its length.

[0020] The length of the throat is preferably approximately equal to itsdiameter.

[0021] Preferably, the point of intersection is between a point locatedabout midway along the length of the throat and the outer end of thethroat.

[0022] The nozzle preferably defines a gas flow path which is alignedwith the axis of the throat, so that gas under pressure can be suppliedto the inlet between the feedstock wires to assist in expulsion ofmolten particles from the outlet.

[0023] The nozzle may define a chamber inwardly of the throat, thechamber having an inner wall which has an average internal diameterseveral times greater than that of the throat and which tapers inwardlytowards an inner end of the throat.

[0024] The inner wall of the chamber preferably joins the inner end ofthe throat at an angle of approximately 45°.

[0025] In the accompanying drawings:

[0026]FIG. 1 is an exploded pictorial view of the front portion of aspray gun according to the invention;

[0027]FIG. 2 is a sectional side view of the nozzle of the spray gun;and

[0028]FIGS. 3a and 3 b are photographs of coatings produced by a priorart arc spray gun and the apparatus of the invention, respectively.

[0029] In the method of the present invention, a high velocity thermalspray gun is used to atomise a feedstock material containing titanium inthe presence of nitrogen to obtain particles comprising titaniumnitride, which are then sprayed onto a substrate to be coated.

[0030] The apparatus of the invention forms part of a spray gun of thiskind, which utilises two or more feedstock wires which are fed throughsuitable guides towards a point of intersection. A suitably highelectrical current is passed through the wires, creating an arc at thepoint of intersection. An air jet atomises the feedstock material, whichis then sprayed onto a substrate.

[0031] In a conventional spray gun of this kind, the feedstock wires arefed through a nozzle, so that their point of intersection is beyond theend of the nozzle. An atomising air jet emitted by the nozzle carriesthe molten particles towards the substrate in a jet.

[0032] In the present invention, the point of intersection of thefeedstock wires is within the throat of the nozzle, rather than outsidethe nozzle. The creation of an arc in the throat has the effect ofgenerating supersonic flow in the nozzle, which would otherwise not beattainable. This very high flow velocity results in very fineatomisation of the molten feedstock particles, and very high particlespeeds as the particles are emitted towards the substrate.

[0033] Referring now to FIGS. 1 and 2, a high velocity spray gunaccording to the invention comprises a nozzle 10 which defines a throat12 in the form of a tubular bore having an inlet 14 and an outlet 16. Inthe prototype apparatus, the length and diameter of the throat wereapproximately equal at 8 mm, with the diameter of the throat beingconstant along its length.

[0034] The interior of the nozzle defines a chamber 18 which has anaverage internal diameter several times greater than that of the throat12 and which is generally frusto-conical in shape. At the end of thechamber adjacent the inlet 14 of the throat 12, the inner wall 20 of thechamber is tapered inwardly more sharply, and joins the inner end of thethroat at an angle of approximately 45°.

[0035] The interior of the nozzle receives a pair of feedstock guides 22and 24 which are inclined towards one another and which are disposedadjacent the inner surface of the chamber 18.

[0036] Wire feedstock material 26 (titanium wire in the basic method ofthe invention) is fed longitudinally thorough the guides 22 and 24 by awire feeder mechanism (not shown), so that the two wires convergetowards a point of intersection located on the axis of the throat 12 ofthe nozzle, between a point approximately midway along the length of thethroat and the outer end of the throat. The dimensions of the throat areselected to permit an arc between the two feedstock wires to be locatedsubstantially within the throat 12.

[0037] In FIG. 1, the included angle between the feedstock guides isabout 30°, but a greater angle, say 60°, leads to a smaller effectivepoint of intersection between the feedstock wires, which is desirable.

[0038] In operation, air (or another nitrogen-rich gas) is forced intothe spray gun head under pressure, with the pressure and volume beingadjusted so that the gas flow within the throat 12 is sonic (i.e.choked) or very close to being choked. Current is applied to thefeedstock wires to create an electric arc between them, so that the airor gas being forced through the throat of the nozzle is heatedsubstantially instantaneously to 4000° C.-5000° C. by the arc. Thisrapid heating of the gas accelerates it to very high velocities,expelling the air and molten feedstock particles from the outlet 16 in afine jet 28.

[0039] In a prototype of the apparatus, a voltage of 35V was appliedbetween the feedstock wires from a constant voltage source, creating anarc current in the region of 180A to 200A. The feed rate of thefeedstock wires was about 3 m/min. A supply of compressed air with apressure of 600 kPa was used, providing a gas pressure in the chamber 18of approximately 400 kPa. The choked pressure in the throat 12 wasapproximately 200 kPa with the throat shape and dimensions given above.

[0040] The feedstock wires have a composition which is selected tocreate a coating having desired chemical and physical characteristics.For example, a 1.6 mm diameter wire of 316 stainless steel can be usedas a feedstock to produce a coating of stainless steel on a substrate.

[0041] Due to the high velocity of the jet, the particles are veryfinely atomised, improving the properties of the coating. Also due tothe high velocity of the jet, the jet is well focused and the deposit itgenerates is very dense.

[0042]FIGS. 3a and 3 b illustrate the difference between coatingsproduced by a conventional arc spray gun and the above describedapparatus of the invention, respectively. The texture of the coatingproduced by the prior art apparatus is relatively coarse, whereas thatproduced by the apparatus of the present invention is much finer andless porous.

[0043] Where titanium is used as a feedstock material, it is believedthat the arc has the effect of ionising the nitrogen (and otherelements) in the air passing through the throat of the nozzle, causing areaction to take place between the nitrogen ions and the molten titaniummetal particles. This results in a high proportion of the titanium metalreacting with the nitrogen to form titanium nitride. In addition,titanium oxide and titanium carbide can be expected to be formed. Due tothe fine atomisation produced by the spray gun, a relatively largepercentage of the atomised titanium metal reacts with the nitrogen, witha resulting large percentage of titanium nitride in the depositedmaterial.

[0044] Coatings formed by the method were found to contain approximately2% to 5% percent of the original titanium metal, which acts as a binderfor the particles of titanium nitride and makes the coating tougher andless brittle. Tests showed that the coatings were very hard, with aVickers hardness of approximately Hv 1100.

[0045] The typical stoichiometery of the coatings referred to above isTi_(1.0)N_(0.94)O_(0.08), which is a titanium nitride compoundcomprising a small proportion of oxygen.

[0046] In order to increase the toughness of the coating formed by themethod of the invention, while retaining the properties of the extremelyhard titanium nitride, a metal selected for its properties as a bindercan be incorporated in the coating. This conveniently achieved byreplacing one of the titanium feedstock wires with a wire of theselected binder metal, for example nickel. The binder metal is thenmixed by the arc spray process with the titanium nitride deposit,producing a composite deposit containing, say, 48% titanium nitride andthe balance comprising the metal, which acts as a binder in the titaniumnitride matrix. The two feedstock wires need not be of exactly the samediameter, thus permitting the percentage of metal binder to titaniumnitride to be varied according to the requirements of the particularapplication.

[0047] A particular advantage of the method of the invention is that itallows the creation of substantially thicker coatings than prior artmethods. Coatings of 0.5 mm thickness or greater are possible. Becausetitanium nitride is chemically inert, the method of the invention isparticularly useful in coating substrates which will be subjected tocorrosive or erosive environments, such as propeller or turbine blades.It is also envisaged that the method will be useful in coating medicalimplants, due to the chemical inertness and biocompatibility of titaniumnitride. The coatings produced by the method also have an attractivegolden colour.

[0048] It was found that, when viewed under high magnification, a largenumber of very small shrinkage cracks (of the order of 0.5 μm) wereexhibited within each spray particle in the deposit or coating. In orderto improve the corrosion protection properties of the coating, a sealersuch as a phenolic resin sealer can be applied, for example by painting,to the coating after spraying. The application of a thin sealant layeronto a titanium nitride coating is particularly effective, as themicro-cracks are extensive and well distributed and the sealer is thuseffectively soaked into the coating, sealing it. Since the sealer isthen contained within the coating matrix, the sealer is protected withinthe coating from mechanical damage, thus ensuring that it is effectivefor an extended period of time.

1. A method of forming a coating on a substrate comprising the steps of:providing a feedstock material containing titanium; atomising thefeedstock material in the presence of nitrogen; and spraying theatomised material onto a substrate to form a coating comprising titaniumnitride on the substrate.
 2. A method according to claim 1 wherein thecoating additionally comprise oxides and carbides of titanium.
 3. Amethod according to claim 1 or 2 wherein the feedstock material isatomised by generating an arc between at least two feedstock elements.4. A method according to claim 3 wherein at least one of the feedstockelements is a titanium wire which is fed towards a point of intersectionbetween the feedstock elements where the arc is generated.
 5. A methodaccording to claim 4 wherein the point of intersection is located withina throat of a nozzle, the method including supplying a nitrogen rich gasunder pressure to the throat of the nozzle to assist in expulsion ofatomised particles therefrom.
 6. A method according to claim 5 whereinthe gas is supplied to the throat of the nozzle at a pressure sufficientto generate choked gas flow in the throat.
 7. A method according toclaim 5 or claim 6 wherein the gas is air.
 8. A method according to anyone of claims 1 to 7 wherein at least one of the feedstock elements is awire comprising a metal selected to have suitable properties as a binderof the titanium nitride in the coating.
 9. A method according to claim 8wherein the metal is nickel.
 10. A method according to any one of claims1 to 9 including the step of applying a protective layer to the coating.11. A method according to claim 10 wherein the protective layercomprises a phenolic resin.
 12. Thermal spraying apparatus comprising: anozzle defining a throat having an inlet and an outlet; at least firstand second guides arranged to guide respective feedstock wires via theinlet towards a point of intersection in the throat, so that connectionof the wires to a power supply causes an arc in the throat between thewires, creating molten particles which are expelled from the outlet. 13.Thermal spraying apparatus according to claim 12 wherein the throatcomprises a tubular bore which substantially surrounds the point ofintersection of the two feedstock wires.
 14. Thermal spraying apparatusaccording to claim 13 wherein the diameter of the throat issubstantially constant along its length.
 15. Thermal spraying apparatusaccording to claim 13 or 14 wherein the length of the throat isapproximately equal to its diameter.
 16. Thermal spraying apparatusaccording to any one of claims 12 to 15 wherein the point ofintersection is between a point located about midway along the length ofthe throat and the outer end of the throat.
 17. Thermal sprayingapparatus according to any one of claims 12 to 16 wherein the nozzledefines a gas flow path which is aligned with the axis of the throat, sothat gas under pressure can be supplied to the inlet between thefeedstock wires to assist in expulsion of molten particles from theoutlet.
 18. Thermal spraying apparatus according to claim 17 wherein thenozzle defines a chamber inwardly of the throat, the chamber having aninner wall which has an average internal diameter several times greaterthan that of the throat and which tapers inwardly towards an inner endof the throat.
 19. Thermal spraying apparatus according to claim 18wherein the inner wall of the chamber joins the inner end of the throatat an angle of approximately 45°.